This invention relates to a novel composition having a liquid crystal structure, and in particular, to a mixture of compounds belonging to the ceramide family, sterol family and a medium suitable for cosmetics or medicaments.
In order to keep smooth skin by adding moisture to the skin and inhibiting abnormal transpiration of moisture from the skin, it is considered that moisture-keeping ability of the stratum corneum of the skin has an important role. Moisture in the stratum corneum contains many water-soluble components such as free amino acids, organic acids, urea and inorganic ions, and they are considered to be concerned in the moisture-keeping of the stratum corneum, so that these substances are practically used for the purpose of improving or preventing dry or rough skin by adding them alone or in combination to cosmetics and external drug preparations for skin use.
Recently, it has been revealed that intercellular lipids in the stratum corneum control transpiration of moisture from the body and keep softness and smoothness of the skin by preventing invasion of stimulants from the outside.
The intercellular lipids are composed of a ceramide as the main component and other components such as cholesterol sulfate, palmitic acid and cholesterol. Regarding a ceramide which corresponds to a racemic compound of general formula (II) that will be described later, preparation of a ceramide composition mixed with the same components described above is disclosed in JP-A-4-327563 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d).
It has been confirmed that such an intercellular lipid in stratum corneum or ceramide composition has high moisture-keeping ability. Taking these circumstances into consideration, ceramides such as racemic ceramide, pseudo-ceramide and glycosyl ceramide have recently been obtained by certain means such as chemical synthesis or extraction from natural materials, and, with the aim of improving conditions of the skin, studies have been positively conducted on the application of foundation cosmetics and medicaments mixed with a lipid composition containing these components to the improvement of dry or rough skin.
Since ceramide is hardly soluble in water or organic solvents, for the purpose of applying ceramide to the skin, it is necessary to use it by firstly preparing a lipid composition having a specified mixing ratio so as to form a lamellar liquid crystal structure, and then effect formation of a lamellar liquid crystal phase and then converting the composition into the form of a cosmetic or medicament by adding pharmacologically acceptable base materials and fillers which do not destroy the liquid crystal phase.
It is considered that intercellular lipid in the stratum corneum is composed of a ceramide as the main component and other components such as cholesterol sulfate, palmitic acid and cholesterol, and their mixture generally forms a lamellar liquid crystal structure.
However, according to the report by Wertz, P. W. et al., uniform lamellar liquid crystal structure cannot be formed from ceramide alone or a mixture of ceramide and cholesterol (Wertz, P. W. et al., J. Invest. Dermatol, vol. 87, p. 582 (1986)).
There is a report to recommend a hair protecting composition containing either ceramide or glycosyl ceramide and at least one kind of cholesterol ester (Japanese Patent 2,510,235).
According to another study, it has been reported that a lipid component was selected from the group consisting of a ceramide, a pseudoceramide, a polyester consisting of a polyol and a fatty acid, a phospholipid, a galactosyldiacylglycerol, a sphingoglycolipid, a derivative of succinic acid and a mixture thereof was used to cure xeroderma (JP-A-8-157283).
Also, there is a report stating that a mixture comprised of a composition having a liquid crystal structure, an amphoteric and/or semi-polar surface active agent, a higher fatty acid and water was prepared and made into a composition which shows excellent cosmetic durability and can improve water resistance and sebum resistance (JP-A-8-217633).
In addition to the above, with the aim of obtaining cosmetics, quasi-drugs or medicaments for use in preventing dry and rough skin, keeping skin moisture to make smooth skin and preventing aged skin, various attempts have been made to apply liquid crystal composed of ceramides and other components such as fatty acids, cholesterol, cholesterol esters, polyhydric alcohols or various lipids.
Their examples include a cosmetic for smooth skin use prepared from a liquid crystal emulsion obtained by heating and mixing at least one compound selected from ceramide, glycosyl ceramide and galactosyl ceramide with cholesterol, a fatty acid, a water-soluble polyhydric alcohol and an aqueous medium (JP-A-7-303473), a liquid crystal skin cosmetic or emulsified skin cosmetic having an effect to prevent or improve dry and rough skin, in which one or more compounds selected from ceramide and a substance having a structure analogous to ceramide, one or more compounds selected from alkylpolyglucosides and one or more compounds selected from alcohols and polyols are formulated (JP-A-9-157149), a cosmetic composition which contains a ceramide-like compound and can delay aging of skin (JP-A-10-226674) and a skin cosmetic having a liquid crystal structure in which a cholesterol ester of an iso or anteiso fatty acid, xcex1-hydroxy acid and sphingolipids such as sphingoglycolipid and sphingophospholipid are formulated (JP-A-11-60461).
In a general way, it is considered that the ceramide or glycosyl ceramide used in each of the above reports is preferably a natural type ceramide rather than a non-natural synthetic ceramide. The natural type ceramide mainly used to date has been obtained from materials such as swine skin, bovine brain, plants and cultured fungi, but they are not satisfactory for users from the viewpoint of purity, cost or unstable supply due to epidemic diseases (e.g., bovine spongiform encephalopathy) of animals to be used.
Under such circumstances, studies on the synthesis of natural type optically active ceramides have been carried out and their supply at an industrial level became possible (JP-A-9-235329).
Among these studies, a lipid composition having a lamellar liquid crystal structure, prepared by formulating a ceramide obtained by chemical synthesis and/or an optical isomer thereof, has been proposed (JP-A-11-12118).
According to this invention, however, the lipid composition is a mixture of four compounds (ceramide, acetylsphinganine, a fatty acid or cholesterol ester and cholesterol), so that it has been required not only to make a lipid composition more simple formulation, but also to show further improved function or performance.
Thus, the invention contemplates providing a lipid composition which can improve dry or rough skin and accompanying diseases by increasing moisture-keeping ability of the stratum corneum to a level higher than the conventional compositions through simple formulation and also providing a cosmetic or medicament containing this lipid composition. This cosmetic or medicament has higher ability to protect the skin from drying by adding appropriate moisture, in comparison with its counterparts in which the conventional lipid compositions are formulated.
With the aim of achieving these objects, the present inventors have conducted intensive studies and found as a result of the efforts that a lipid composition shown below, namely a lipid composition comprised of two types of synthetic ceramide and cholesterol, forms a lamellar liquid crystal structure by a blending ratio described in the following. The invention has been accomplished based on this finding.
The lipid composition of the invention is comprised of a racemic 2-acylaminoalkane-1,3-diol or an optically active compound thereof, a racemic 2-acylaminoalkane-1,3-diol having an acyl group in which at least one of the xcex1-position and xcex2-position is substituted with hydroxyl group or an optically active compound thereof and sterols.
This composition can further contain a pharmaceutically acceptable base material or filler. Such a composition is suitably used particularly in, e.g., cosmetics, external skin protection preparations and bath additives.
This composition can keep moisture in those to which it is applied, and when applied to the skin, it prevents abnormal transpiration of moisture from the skin and also adds appropriate humidity to the skin. Thus, the composition of the invention can protect the skin from drying and its appropriate formulation can provide, e.g., desirable cosmetics, external skin protection preparations or bath additives.
Accordingly, the invention is described as follows.
1. A lipid composition which comprises (1) at least one component (A) selected from the group consisting of a 2-acylaminoalkane-1,3-diol represented by a general formula (I) 
(wherein R1 represents a straight chain alkyl group having from 9 to 17 carbon atoms and R2 represents a straight chain acyl group having from 14 to 24 carbon atoms) and optically active compounds thereof, (2) at least one component (B) selected from the group consisting of a 2-acylaminoalkane-1,3-diol represented by a general formula (II) 
(wherein R1 represents a straight chain alkyl group having from 9 to 17 carbon atoms and R3 represents a straight chain acyl group having from 2 to 24 carbon atoms in which at least one of the xcex1-position and xcex2-position is substituted with hydroxyl group) and optically active compounds thereof and (3) at least one component (C) selected from sterols, wherein the components (A), (B) and (C) are mixed at a ratio that constructs a liquid crystal structure.
2. The lipid composition having a liquid crystal structure according to the above item 1, wherein the sterols of component (C) is cholesterol.
3. The lipid composition having a liquid crystal structure according to the above item 1 or 2, wherein the component (A) is a (2S,3R)-2-acylaminoalkane-1,3-diol represented by a general formula (III) 
(wherein R1 and R2 are as defined in the foregoing).
4. The lipid composition having a liquid crystal structure according to any one of the above items 1 to 3, wherein the component (B) is a (2S,3R)-2-acylaminoalkane-1,3-diol represented by a general formula (IV) 
(wherein R1 and R3 are as defined in the foregoing).
5. The lipid composition having a liquid crystal structure according to any one of the above items 1 to 4, wherein R3 of the component (B) is an acyl group in which the xcex1-position carbon of xcex2-position carbon is substituted with hydroxyl group.
6. The lipid composition having a liquid crystal structure according to any one of the above items 1 to 5, wherein weight ratios of the component (A) to component (B), component (B) to component (C) and component (A) to component (C) are within the range of from 1:9 to 9:1, from 9:6 to 1:10 and from 1:1 to 1:9, respectively.
7. A cosmetic or medicament which comprises the lipid composition defined in any one of the above items 1 to 6 containing a pharmaceutically acceptable base material as an additional component.
Other objects and advantages of the invention will be made apparent as the description progresses.